1. Field of the Invention
The present invention relates to a multi-layer printed circuit board provided with a means for absorbing noise in an image forming apparatus.
2. Description of the Related Art
The performance of image forming apparatus, such as copiers or the like have recently been enhanced and they have been made more compact, resulting in more complicated electronic circuits used in the apparatus. Therefore, the apparatus employs a printed circuit board which is compactly composed and is multi-layered.
In image forming apparatus, however, there are noise-generating sources such as plural high voltage power units, motors, plungers, relays and others are locate therein. Therefore, when a CPU (Central Processing Unit) controls a sequence of image forming, the control is disturbed by the noise of the noise-generating sources, resulting in possible problems. In particular, the influence of noise is more serious on printed circuit boards in a photoreceptor drum which tends to be located in the vicinity of a high voltage noise source. Therefore, noise-preventing items such as capacitors, coils and EMI filters are connected to the electronic circuit related to the noise source.
As stated above, the effect of noise prevention by means such as capacitors, filters and others depends practically on the configuration of a printed circuit board and on the kinds of parts to be used. Accordingly, many parts are connected on a trial and error basis on each occasion, resulting in a cost increase.
Further, a multi-layer printed circuit board makes it difficult to incorporate additional parts later, and the printed circuits mutual interference is not only two dimensional but three dimensional, resulting generally in a drastic fall in the level of noise resistance. For example, in the case of a normal printed circuit board having circuits on both sides, the distance between both circuits is about 1.6 mm, while on a multi-layer printed circuit board, the distance is about 50 .mu.m which is less than one hundredth of the normal circuit distance. Therefore, it has been confirmed that anti-noise property falls drastically from 5 locations on a double-sided printed circuit board down to "locations on a multi-layer printed circuit board, when they are compared in terms of the number of location points where an error is caused by a given prescribed noise signal at 150 locations. Further, in a comparison of discharge noise resistance, measurements of arcing in the vicinity of a printed circuit board have proved that a double-sided printed circuit board can withstand up to as much as 4 KV, while a multi-layered printed circuit board can withstand up to 2 KV at the most. Thus, a conventional multi-layered printed circuit board has many problems.